Add the SCP-firmware framework

Please see doc/framework.md for details.

Co-authored-by: Filipe Rinaldi <filipe.rinaldi@arm.com>
Co-authored-by: Paul Beesley <paul.beesley@arm.com>
Co-authored-by: Chris Kay <chris.kay@arm.com>
Co-authored-by: Elieva Pignat <elieva.pignat@arm.com>
Co-authored-by: Pedro Custodio <pedro.krewinkelcustodio@arm.com>

Change-Id: I1744c60eca295c60b3fabcfafb522fbff5d7306c
Signed-off-by: Ronald Cron <ronald.cron@arm.com>